Polysulfide (PS) is a pulping additive which has been used commercially to increase pulping yield. A higher pulping yield improves process economics by decreasing wood consumption and/or increasing pulp throughput. Polysulfide is commercially produced by catalytic oxidation of part of the sulfide ions contained in Kraft pulping alkali solution, often called “white liquor” in the art of Kraft pulping. This oxidation process is currently the most commercially viable technology that converts sulfide in white liquor to polysulfide, giving the resultant liquor an orange color. Polysulfide alkali liquor thus is also called “orange liquor” in the art.
Polysulfide is found to be effective in increasing pulping yield only when it is applied to the beginning of a cook, e.g., to an impregnation stage where the temperature is typically below ˜140° C. (˜284° F.) and a retention time of typically 15-45 minutes. At or above ˜140° C. (˜284° F.), polysulfide starts to decompose rapidly and loses its effectiveness as a pulping yield enhancer. Pulping yield increase from polysulfide pulping is found to increase proportionately with amounts of polysulfide added to the beginning of a cook (up to about 7% polysulfide charged on wood). Thus in polysulfide pulping, all polysulfide liquor (orange liquor) is most preferably added to the beginning of a cook so as to maximize pulping yield increase. This feature works well with conventional Kraft pulping. In conventional Kraft pulping, which had been the only commercial practice until the late 1970s, the total alkali charge required for a cook is added to the beginning of the cook.
In modified Kraft pulping (modified cooking) developed in the late 1970s, the total alkali charge is divided into at least two and often more than two additions. Typically, only about 45-75% of the total alkali is added to the beginning of a modified cook. By splitting the total alkali charge into several additions to different cooking stages, alkali concentration profile in modified cooking is more even throughout the cook than in conventional Kraft cooking. Of particular importance is the concentration of effective alkali (EA) in the early cooking stage, where the cooking temperature goes from an impregnation temperature of typically ≦135° C. (≦275° F.) to full cooking temperature, typically between 150 to 175° C. (302 to 347° F.). When the EA concentration is too high in this early cooking stage, excessive losses occur in pulping yield and pulp strength. Therefore, modified cooking with a more even alkali profile, particularly a lower EA concentration in the early cooking stage, results in significantly higher pulping yield and pulp strength than conventional Kraft pulping, where the total alkali charge is all added to the beginning of a cook and the EA concentration is high at the early stage.
However, when current commercial polysulfide pulping technology is applied to modified cooking, only 45-75% of the total available polysulfide is added to the beginning of a cook, since only 45-75% of the polysulfide-containing alkali liquor is added to the beginning of the cook. As a result, compared to conventional cooking with polysulfide, only a fraction of the total pulping yield increase is realized because the yield increases are proportional to the amount s of polysulfide added to the beginning of a cook as discussed before. This means that in the prior art, current modified cooking cannot take full advantage of polysulfide pulping for maximum yield increases. In other words, the current modified cooking technology is not completely compatible with the current commercial polysulfide pulping technology.
The present invention overcomes the aforementioned incompatibility of modified Kraft pulping with current commercial polysulfide pulping technology. It obtains all benefits of modified cooking as compared to conventional cooking, and the full yield improvement of polysulfide pulping.